Abstract Good nutritional practices are essential to support the high milk production of modern dairy cows. This requires an active rumen microbial biomass which supplies microbial protein and short-chain fatty acids to the cow. Glycal Forte® (GF) a calcium-stabilised glycerol product, increased microbial biomass in an in vitro gas production study, by some 37%. The efficacy of GF was substantiated in a trial with high-producing dairy cows fed a TMR with or without GF for 28 d. Cows fed GF showed behaviour differences. Chewing was reduced from 710 GF vs 769 for the control (CON) min/d; P=0.01 and they spent more time in an inactive state (429 GF vs 383 CON min/d; P=0.01). There was a statistically significant improvement in feed efficiency with GF (2.19 CON vs 2.34 GF kg ECM/kg dry matter intake (DMI); P=0.03). Feed efficiency has an important bearing on costs and profitability of a dairy operation. Cows fed GF yielded an improved profit of $1.45 per cow per day over 28 d with an ROI of 2.23. Extrapolated to a 100 dairy cow herd for 28 d gave an additional profit of $4,060. Milk yields continued to show benefits for 70 d in milk (DIM) after the GF treatment was stopped, showed a numerical increase in milk yield of 3.2 kg/d. The benefits in rumen function continues after the feeding of GF had ceased. In summary, this trial with high-producing dairy cows was consistent with the in vitro trial data and showed that GF can improve rumen function manifested as less time chewing, more inactive time and most importantly, improved feed efficiency. This has both economic and environmental advantages in that dairy cow productivity can be enhanced solely by nutritional means
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